Numerous studies are concerned with the social and behavioral development of monkeys. Much of the work falls into three categories: descriptive developmental research, manipulations of rearing environments, and parametric studies of variables thought to influence social development. The descriptive research emphasizes development in socially adequate environments with a minimum of experimental intervention. The rearing environment research involves assessing the effects of socially impoverished rearing environments on monkey development and ultimate social capacity. The investigation of variables influencing social development includes studies of factors which significantly affect an infant monkey's attachment to a cloth mother-surrogate. Special interest is also taken in other forms of relationships involving attachment or affection. Emphasis is placed on understanding basic mechanisms underlying social development, and comprehending social bonds among preadolescent and adolescent monkeys. In the past, some studies have been concerned with the effects of brain lesions on behavior. In 1981, the individual variation in the social/emotional development of young monkeys continues to be studied. The basic research paradigm involves manipulating infant temperament via selective breeding procedures, manipulating maternal rejection by cross fostering with mothers (or artificial surrogates) displaying different rejection rates, and exposing infants to these manipulations in rearing environments that differ in availability of nonmaternal conspecifics and physical restrictiveness. The developmental continuity and stability of individual differences in autonomic reactivity, neuroendocrine activity, and social behavior is determined.